# Why would a black hole have a disk, but not emit x-rays?

There's a recent paper in Nature about LB-1, a B-class star orbiting a massive black hole.

I don't understand how these two parts of the paper can be reconciled. On page 2, the authors argue that the broad $$H\alpha$$ line indicates there's a disk in the system, and then that the disk is around the black hole (as opposed to the star or the entire binary).

This supports the $$H\alpha$$ emission line not coming from a circumbinary disk, but from a disk around the black hole.

However in the last paragraph, they also say that this system doesn't emit X-rays:

Unlike every other known stellar black hole, LB-1 has not been detected in X-ray observations. We searched for X-ray emission from this system with a 10-ks observation with the Chandra X-ray Observatory, placing an upper limit for the X-ray luminosity of $$\geq 2 × 10^{31} erg s^{−1}$$ (see Methods). This upper limit corresponds to about $$10^{−9}$$ of its Eddington luminosity, and suggests a mass accretion rate ̇$$M \leq 10^{-11} M_{sun} yr^{-1}$$ for a conversion efficiency of approximately $$10^{−4}$$ at such low luminosity.

Naively it seems to me that this non-detection is actually an argument that the $$H\alpha$$ line isn't coming from a disk around the black hole. If there's a disk around a black hole, it's presumably accreting. If it's accreting, the material presumably gets hot and produces X-rays. If it's producing X-rays, Chandra should have detected them.

What am I missing?

• Short answer: because there is very little friction in the particular disk, leading to a low accretion rate. (Not an answer, because it is just shifting the problem.) – mmeent Nov 28 '19 at 15:58
• I'll note that there are at least three independent preprints out today arguing that the analysis was incorrect, with signs that the $\text{H}\alpha$ emission is not, in fact, coming from such a putative disk (e.g. arxiv.org/abs/1912.04185). Seems like the original group might have neglected to properly model the stellar companion's $\text{H}\alpha$ emission features, which might cast doubt on the disk's existence. – HDE 226868 Dec 10 '19 at 2:52